Occasionally, I am asked to play on someone else's recording, and I quite enjoy the session/sideman gigs when they come along. The pieces are usually completely written when I get to play on them, so all I have to do is play my parts under the direction of the producer and/or composer, get it how they want it, and not take too long to do it - in the studio, time is money in a very real sense, and it definitely works in your favour if you can get a part down in a couple of takes.
These days, the people who hire me know what they're getting, so it is rarely that I find myself on unfamiliar ground, music-wise. If a project requires a guitarist with a deep knowledge of Jazz, chances are my name won't come up. Rock, Blues and Pop have been the main gigs that I get the call for, and I approach each session based on its requirements - not every song needs a screaming guitar solo, and some songs require none at all. Working with the producer/composer to find the best thing for the song is always an interesting task, and no two sessions are the same. I can see why full-time session players love their gig.
The first thing I do after hearing the tracks I'm to play on is to find out how much of 'me' I'm supposed/allowed to put in to the track. Sometimes the guitar parts are quite definite, and I play what's written, but just as often I am asked to "just go for it" in solo sections, or to add my own feel to a part - one of the upsides of being known for a particular style of playing. Once I have my parameters set, I get to work and get all my parts under my fingers and committed to memory, keeping in mind that on-the-spot changes may happen as circumstances or inspiration may dictate. If I know I have the parts together as best I can, I can approach the session in a relaxed manner, which is always much more fun than stressing over whether or not I can remember everything. Preparation, like pre-production, is the key to a good session, and will make the callbacks for other jobs more likely if taken seriously.
I also enjoy the freedom of not having my name on the work, aside from recording credits. While I always give my best, I am not in the firing line of opinion and response to the product, so I can relax after the job is done without sweating on releases and sales. I can enjoy the music for what it is, not having to worry about what other people think about it. Having said that, it's always good to see something you've played on perform well, and (for me, anyway) hearing a piece can invoke memories of the session and what happened on the day the tracks went down. Some sessions are in-and-out affairs, but others are exercises in sharing a unique experience with talented and beautiful people who care about what's happening in the room. Those times are the ones that stick in the memory for years, if not life. I guess that's a big part of why I still do this music thing, and still love it.